The present invention is generally directed to a method for modulation, particularly amplitude modulation, of light by a Bragg modulator. In particular, the present invention is directed to a method and apparatus for recording a product to be printed, preferably for the manufacture of printed forms, involving at least one Bragg modulator on the basis of domain-invertible material.
Employing a Bragg modulator on the basis of domain-inversion for recording a product to be printed, is fundamentally known from U.S. Pat. No. 6,025,864, namely as a component part of a scanner.
In addition, utilizing fiber lasers for manufacturing printing forms is known from DE-A-198 40 926.
It is well known that fiber lasers output unpolarized light. Using Bragg modulators for the light from fiber lasers, or for other unpolarized light, seems problematical because the index of diffraction of the diffraction grating generated by an electrical voltage and changed due to domain-inversion, acts on only one polarization direction or in one polarization plane. An acousto-optical device is therefore usually used for the modulation of unpolarized light.
It is an object of the invention to provide a method and an apparatus wherein a Bragg modulator can be used to modulate unpolarized light.
It is a further object of the present invention to use a first Bragg modulator to modulate a light part of unpolarized light in a polarization plane defined by its orientation.
It is another object of the invention to use a second Bragg modulator to modulate the light part of unpolarized light to be assigned to a second polarization plane orthogonal to a first polarization plane.
It is a further object of the invention to modulate light independently of its polarization planes, so that an acousto-optical modulator is not needed.
Using Bragg modulators for modulating unpolarized light in this way has many advantages. For one thing, modulation is possible with high efficiency such as on the order of magnitude of about 99%. For another thing, the efficiency and the modulation speed are advantageously not fundamentally interdependent. In addition, the laser beam can have a relatively large beam diameter, even given high modulation speeds. Since the beam diameter can be large, the power density in the crystal of the modulator can be relatively low. It in turn follows that fiber lasers can be used for manufacturing printing forms.
The two Bragg modulators are situated such that, for two mutually orthogonal polarization planes of the unpolarized light, the modulators are arranged following one another on the light path.
For the modulators to act on the two polarization planes of the light, the light between the modulators can be turned by 90xc2x0 with a xcex/2 plate, and the modulators themselves can be arranged with their gratings parallel to one another, or the modulators can be arranged relative to one another with their gratings turned by 90xc2x0 around the beam axis of the light. Arranging gratings in parallel, allows a planar structure of the modulator unit.
Exemplary embodiments, from which further inventive features also derive but that do not limit the invention, are shown in the drawings.